Keeping My Temper Under Control

Lynne Bulhack, Ceramist, Experimental Archaeologist, ASM Native American Liaison

In the context of Indigenous ceramics, temper is a substance added to clay that modifies its

properties when wet or dry as well as during and after firing and in use. A variety of materials

may be added to clay when preparing to make pottery; organic materials of plant or animal origin, mineral tempers in the form of crushed rock and sand, as well as material of human activity in the form of crushed pot sherds (referred to as grog). Archeological literature suggests the properties these materials modify include: 1) reducing shrinkage in drying and firing, 2) lowering the vitrification temperature of the paste, 3) increasing fired strength, and 4) increasing or decreasing plasticity. Through ceramic experimentation, I have found that only mixing low plasticity clay with a more plastic clay will increase plasticity of the paste and adding fine particles of soapstone will dramatically decrease plasticity. In an effort to improve clay plasticity, traditional Japanese potters employed a series of settling ponds making use of sedimentation to separate fine material from coarse material by suspension in water. In 28 years of teaching ceramics, I have had only one student successfully use sedimentation to slightly improve the plasticity of "wild" clay.

Temper in archeologically recovered sherds and pots is a primary attribute in defining an

Indigenous ceramic type along with surface treatment, morphology, and decoration. A typology often becomes a main unit of analysis in developing chronologies, geographic distributions, indicators of technological change, trade activity, and even migration of peoples. It is important to recognize that a sherd or a group of sherds may not be representative of a whole pot. This brief essay will not address the questions raised by using typed sherds to tell these stories. I will focus on hands in the mud empirical experience; experimental archeology.

Stallings Island plant fiber-tempered pottery is the oldest known pottery in North America dating to around 2,500 BC. I have not experimented with plant- fiber temper. Crushing

rock, shell and grog with my favorite hammer-stone results in large, medium, and small chunks of rock and shell as well as powder (Figure 1).

Figure 1. Crushed oyster shell with a hammer stone.

I generally keep pounding until I achieve what I determine is the best particle size for the shape, size, and function of the intended pot. I often wonder if Indigenous folks sieved their crushed material when adding temper to the clay? (Figure 2).

Figure 2. Experimentally Crushed Tempers.

The majority of Indigenous pots were intended for daily use in storage, food preparation, serving, and the processing of animal and plant material. Quartz and grog crush easily. Hardness of the limestone and steatite or soapstone that I found was highly variable. I did not do controlled heating experiments to determine what temperatures were needed to improve crushability, I simply found softer varieties of limestone and soapstone.

Steatite or soapstone was the least agreeable temper I used because it is primarily composed of the hydrous, magnesium, silicate mineral talc. Crushing soapstoneto an acceptable temper size resulted in an abundance of powder. The powder along with the chunks added to the clay paste immediately absorbed a significant amount of water greatly reducing plasticity. Much messing about with water and temper ratios was required to get back to a cooperative, plastic clay paste. Here, I must say that as a maker of pottery, someone who finds clay magical and inspirational, I am appalled at using the word paste to refer to the weathered rock minerals that become clay and when given form and heated to various temperatures above 500 degrees C. once again becomes rock-like. I have

had test pots fired below 500 degrees C. return to mud in a spring rain. I do not make pots with clay "paste". I use a clay "body" or clay "matrix".

It is important to keep in mind that "wild" clays are highly variable and the same temper in

different clays will behave differently. I have been invited to Williamsburg by my friend and

colleague Daniel Abbott, Nanticoke interpreter of Native American life ways, to do some

experimental firing of shell tempered pottery using Pamunkey clay. Experiments with small amounts of Pamunkey clay have shown that the low temperatures common in bon-firing will render Pamunkey clay hard enough to ring like a bell when struck with a metal utensil unlike other clays fired to the very same temperature.

Calcium carbonate tempers such as marine shell impose stringent limits on the firing temperature of shell tempered pottery. The calcium carbonate of shell decomposes at temperatures between 650 degrees C. and 898 degrees C. and becomes calcium oxide. Calcium oxide in the fired ceramic takes up atmospheric moisture after firing, causing parts of the vessel to crumble. That is exactly what happened to my first firing of shell tempered pots. A shorter, faster firing where the temperature of the pots did not exceed 750 degrees C. solved the problem with that particular clay and its shell temper. Temperatures and atmosphere within a bonfire vary. What is important to pots in a firing is heat work which is a combination of time and temperature and cannot be accurately measured with a pyrometer. Indigenous potters could read their pots during the important preheating phase of firing, understood their fuel and could read the fire, intimacy with material and process enabled success. Indigenous knowledge is not, was not primitive.